# Svrnty.MCP.Gateway - API Reference

**Version:** 1.0.0
**Last Updated:** 2025-10-19
**Status:** Production-Ready

---

## Table of Contents

- [Core Abstractions](#core-abstractions)
  - [IGatewayRouter](#igatewayrouter)
  - [IRoutingStrategy](#iroutingstrategy)
  - [ICircuitBreaker](#icircuitbreaker)
- [Infrastructure](#infrastructure)
  - [GatewayRouter](#gatewayrouter)
  - [RoutingStrategies](#routing-strategies)
  - [CircuitBreaker](#circuitbreaker)
- [Models](#models)
  - [RoutingContext](#routingcontext)
  - [ServerInfo](#serverinfo)
  - [GatewayConfig](#gatewayconfig)
- [ASP.NET Core Integration](#aspnet-core-integration)
  - [Service Extensions](#service-extensions)
  - [Endpoint Mapping](#endpoint-mapping)
- [Monitoring](#monitoring)

---

## Core Abstractions

### IGatewayRouter

**Namespace:** `Svrnty.MCP.Gateway.Core.Abstractions`

Interface defining the gateway routing contract.

#### Methods

##### RouteRequestAsync

```csharp
Task<McpToolResult> RouteRequestAsync(
    RoutingContext context,
    CancellationToken cancellationToken = default)
```

Routes an MCP request to an appropriate backend server.

**Parameters:**
- `context` (RoutingContext): Request routing context
- `cancellationToken` (CancellationToken): Cancellation support

**Returns:** `Task<McpToolResult>` - The result from the backend server

**Throws:**
- `NoHealthyServersException` - If all servers are unhealthy
- `RoutingException` - If routing decision fails
- `CircuitBreakerOpenException` - If circuit breaker is open

**Example:**
```csharp
var context = new RoutingContext
{
    ClientId = "web-client",
    ToolName = "search_documents",
    Parameters = new Dictionary<string, object>
    {
        ["query"] = "architecture"
    }
};

var result = await router.RouteRequestAsync(context);
```

##### GetServerHealthAsync

```csharp
Task<IEnumerable<ServerHealth>> GetServerHealthAsync(
    CancellationToken cancellationToken = default)
```

Gets health status of all registered servers.

**Returns:** `Task<IEnumerable<ServerHealth>>` - Health information for each server

**Example:**
```csharp
var healthStatuses = await router.GetServerHealthAsync();
foreach (var status in healthStatuses)
{
    Console.WriteLine($"{status.ServerName}: {status.Status}");
}
```

---

### IRoutingStrategy

**Namespace:** `Svrnty.MCP.Gateway.Core.Abstractions`

Interface for implementing custom routing logic.

#### Methods

##### SelectServer

```csharp
string SelectServer(
    RoutingContext context,
    IEnumerable<ServerInfo> availableServers)
```

Selects a server to handle the request.

**Parameters:**
- `context` (RoutingContext): Request context
- `availableServers` (IEnumerable<ServerInfo>): Healthy servers

**Returns:** `string` - ID of the selected server

**Throws:**
- `NoServerAvailableException` - If no servers match criteria

**Example Implementation:**
```csharp
public class ToolBasedRoutingStrategy : IRoutingStrategy
{
    public string SelectServer(
        RoutingContext context,
        IEnumerable<ServerInfo> servers)
    {
        // Route based on tool name pattern
        if (context.ToolName.StartsWith("search_"))
        {
            return servers.First(s => s.Name == "search-server").Id;
        }

        // Default: first available
        return servers.First().Id;
    }
}
```

---

### ICircuitBreaker

**Namespace:** `Svrnty.MCP.Gateway.Core.Abstractions`

Interface for circuit breaker pattern implementation.

#### Properties

##### State

```csharp
CircuitBreakerState State { get; }
```

Current state of the circuit breaker.

```csharp
public enum CircuitBreakerState
{
    Closed,   // Normal operation
    Open,     // Failing, rejecting requests
    HalfOpen  // Testing if recovered
}
```

#### Methods

##### IsOpen

```csharp
bool IsOpen(string serverId)
```

Checks if circuit breaker is open for a specific server.

**Parameters:**
- `serverId` (string): Server identifier

**Returns:** `bool` - True if circuit is open (rejecting requests)

##### RecordSuccess

```csharp
Task RecordSuccessAsync(string serverId)
```

Records a successful request.

##### RecordFailure

```csharp
Task RecordFailureAsync(string serverId, Exception exception)
```

Records a failed request.

**Example:**
```csharp
if (_circuitBreaker.IsOpen("server-1"))
{
    throw new CircuitBreakerOpenException("server-1");
}

try
{
    var result = await CallServerAsync("server-1", request);
    await _circuitBreaker.RecordSuccessAsync("server-1");
    return result;
}
catch (Exception ex)
{
    await _circuitBreaker.RecordFailureAsync("server-1", ex);
    throw;
}
```

---

## Infrastructure

### GatewayRouter

**Namespace:** `Svrnty.MCP.Gateway.Infrastructure`

Default implementation of `IGatewayRouter`.

#### Constructor

```csharp
public GatewayRouter(
    IRoutingStrategy routingStrategy,
    ICircuitBreaker circuitBreaker,
    IServerRegistry serverRegistry,
    ILogger<GatewayRouter> logger = null)
```

**Parameters:**
- `routingStrategy` (IRoutingStrategy): Strategy for server selection
- `circuitBreaker` (ICircuitBreaker): Circuit breaker implementation
- `serverRegistry` (IServerRegistry): Registry of backend servers
- `logger` (ILogger): Optional logger

**Example:**
```csharp
var registry = new ServerRegistry();
registry.AddServer(new ServerInfo
{
    Id = "server-1",
    Name = "CODEX Server",
    BaseUrl = "http://localhost:5050"
});

var router = new GatewayRouter(
    new RoundRobinStrategy(),
    new CircuitBreaker(),
    registry
);
```

---

### Routing Strategies

Built-in routing strategy implementations.

#### RoundRobinStrategy

Distributes requests evenly across all healthy servers.

```csharp
public class RoundRobinStrategy : IRoutingStrategy
{
    public string SelectServer(
        RoutingContext context,
        IEnumerable<ServerInfo> servers)
    {
        // Round-robin logic
        var serverList = servers.ToList();
        var index = _counter++ % serverList.Count;
        return serverList[index].Id;
    }
}
```

**Use Case:** Load balancing across identical servers

#### LeastConnectionsStrategy

Routes to the server with fewest active connections.

```csharp
public class LeastConnectionsStrategy : IRoutingStrategy
{
    public string SelectServer(
        RoutingContext context,
        IEnumerable<ServerInfo> servers)
    {
        return servers
            .OrderBy(s => s.ActiveConnections)
            .First()
            .Id;
    }
}
```

**Use Case:** Balancing load when servers have different capacities

#### ToolBasedStrategy

Routes based on tool name patterns.

```csharp
public class ToolBasedStrategy : IRoutingStrategy
{
    private readonly Dictionary<string, string> _toolToServerMap;

    public string SelectServer(
        RoutingContext context,
        IEnumerable<ServerInfo> servers)
    {
        if (_toolToServerMap.TryGetValue(context.ToolName, out var serverId))
        {
            return serverId;
        }

        // Default fallback
        return servers.First().Id;
    }
}
```

**Use Case:** Route specific tools to specialized servers

---

### CircuitBreaker

**Namespace:** `Svrnty.MCP.Gateway.Infrastructure.Resilience`

Default circuit breaker implementation.

#### Configuration

```csharp
public class CircuitBreakerConfig
{
    public int FailureThreshold { get; set; } = 5;
    public int SuccessThreshold { get; set; } = 2;
    public TimeSpan Timeout { get; set; } = TimeSpan.FromMinutes(1);
}
```

#### Usage

```csharp
var config = new CircuitBreakerConfig
{
    FailureThreshold = 5,    // Open after 5 failures
    SuccessThreshold = 2,    // Close after 2 successes
    Timeout = TimeSpan.FromMinutes(1)  // Test recovery after 1 min
};

var circuitBreaker = new CircuitBreaker(config);
```

---

## Models

### RoutingContext

**Namespace:** `Svrnty.MCP.Gateway.Core.Models`

Context information for routing decisions.

#### Properties

```csharp
public class RoutingContext
{
    public string ClientId { get; set; }
    public string ToolName { get; set; }
    public Dictionary<string, object> Parameters { get; set; }
    public Dictionary<string, string> Headers { get; set; }
    public DateTime Timestamp { get; set; }
}
```

#### Example

```csharp
var context = new RoutingContext
{
    ClientId = "web-client-123",
    ToolName = "search_documents",
    Parameters = new Dictionary<string, object>
    {
        ["query"] = "test"
    },
    Headers = new Dictionary<string, string>
    {
        ["X-Client-Version"] = "1.0.0"
    },
    Timestamp = DateTime.UtcNow
};
```

---

### ServerInfo

**Namespace:** `Svrnty.MCP.Gateway.Core.Models`

Information about a registered backend server.

#### Properties

```csharp
public class ServerInfo
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string BaseUrl { get; set; }
    public bool IsHealthy { get; set; }
    public int ActiveConnections { get; set; }
    public DateTime LastHealthCheck { get; set; }
    public TimeSpan AverageResponseTime { get; set; }
}
```

#### Example

```csharp
var serverInfo = new ServerInfo
{
    Id = "codex-server-1",
    Name = "CODEX MCP Server 1",
    BaseUrl = "http://localhost:5050",
    IsHealthy = true,
    ActiveConnections = 3,
    LastHealthCheck = DateTime.UtcNow,
    AverageResponseTime = TimeSpan.FromMilliseconds(45)
};
```

---

### GatewayConfig

**Namespace:** `Svrnty.MCP.Gateway.Core.Models`

Gateway configuration.

#### Properties

```csharp
public class GatewayConfig
{
    public string Name { get; set; }
    public string Version { get; set; }
    public string ListenAddress { get; set; }
    public List<ServerConfig> Servers { get; set; }
    public RoutingConfig Routing { get; set; }
    public SecurityConfig Security { get; set; }
}

public class ServerConfig
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string BaseUrl { get; set; }
    public bool Enabled { get; set; }
}

public class RoutingConfig
{
    public string Strategy { get; set; }  // "RoundRobin", "LeastConnections", "ToolBased"
    public TimeSpan HealthCheckInterval { get; set; }
}
```

---

## ASP.NET Core Integration

### Service Extensions

**Namespace:** `Svrnty.MCP.Gateway.AspNetCore`

#### AddMcpGateway

```csharp
public static IServiceCollection AddMcpGateway(
    this IServiceCollection services,
    IConfiguration configuration)
```

Registers gateway services and dependencies.

**Configuration:**
```json
{
  "Mcp": {
    "Gateway": {
      "Name": "MyGateway",
      "Version": "1.0.0",
      "ListenAddress": "http://localhost:8080"
    },
    "Servers": [ ... ],
    "Routing": {
      "Strategy": "RoundRobin",
      "HealthCheckInterval": "00:00:30"
    }
  }
}
```

**Example:**
```csharp
var builder = WebApplication.CreateBuilder(args);

builder.Services.AddMcpGateway(
    builder.Configuration.GetSection("Mcp")
);

builder.Services.AddHealthChecks()
    .AddCheck<McpServerHealthCheck>("mcp-servers");

var app = builder.Build();
app.MapMcpGateway();
app.MapHealthChecks("/health");
app.Run();
```

---

### Endpoint Mapping

#### MapMcpGateway

```csharp
public static IEndpointRouteBuilder MapMcpGateway(
    this IEndpointRouteBuilder endpoints)
```

Maps gateway HTTP endpoints.

**Endpoints:**
- `POST /mcp/invoke` - Route MCP requests
- `GET /health` - Gateway health check
- `GET /servers` - List backend servers
- `GET /servers/{id}/health` - Server-specific health

**Example:**
```csharp
var app = builder.Build();
app.MapMcpGateway();
```

---

## Monitoring

### Metrics

OpenTelemetry metrics exposed by the gateway:

```csharp
public class GatewayMetrics
{
    public Counter<long> TotalRequests { get; }
    public Histogram<double> RequestDuration { get; }
    public Counter<long> TotalErrors { get; }
    public Gauge<int> ServerHealth { get; }
    public Gauge<int> CircuitBreakerState { get; }
}
```

**Metric Names:**
- `mcp_gateway_requests_total`
- `mcp_gateway_request_duration_ms`
- `mcp_gateway_errors_total`
- `mcp_gateway_server_health`
- `mcp_gateway_circuit_breaker_state`

**Example Query (Prometheus):**
```promql
# Average request latency
rate(mcp_gateway_request_duration_ms_sum[5m]) /
rate(mcp_gateway_request_duration_ms_count[5m])

# Error rate
rate(mcp_gateway_errors_total[5m]) /
rate(mcp_gateway_requests_total[5m])
```

---

### Health Checks

#### Gateway Health Endpoint

```bash
curl http://localhost:8080/health
```

**Response:**
```json
{
  "status": "Healthy",
  "totalServers": 3,
  "healthyServers": 3,
  "degradedServers": 0,
  "unhealthyServers": 0,
  "servers": [
    {
      "id": "codex-server-1",
      "name": "CODEX Server 1",
      "status": "Healthy",
      "lastCheck": "2025-10-19T12:00:00Z",
      "responseTime": "12ms"
    }
  ]
}
```

#### Custom Health Check

```csharp
public class McpServerHealthCheck : IHealthCheck
{
    private readonly IGatewayRouter _router;

    public async Task<HealthCheckResult> CheckHealthAsync(
        HealthCheckContext context,
        CancellationToken cancellationToken = default)
    {
        var serverHealth = await _router.GetServerHealthAsync(cancellationToken);

        var unhealthyServers = serverHealth
            .Count(s => s.Status != ServerHealthStatus.Healthy);

        if (unhealthyServers == 0)
        {
            return HealthCheckResult.Healthy("All servers healthy");
        }

        if (unhealthyServers == serverHealth.Count())
        {
            return HealthCheckResult.Unhealthy("All servers unhealthy");
        }

        return HealthCheckResult.Degraded(
            $"{unhealthyServers} of {serverHealth.Count()} servers unhealthy"
        );
    }
}
```

---

## Complete Example

### Creating a Gateway

```csharp
using Svrnty.MCP.Gateway.AspNetCore;
using Svrnty.MCP.Gateway.Core;

var builder = WebApplication.CreateBuilder(args);

// Add gateway services
builder.Services.AddMcpGateway(
    builder.Configuration.GetSection("Mcp")
);

// Add custom routing strategy
builder.Services.AddSingleton<IRoutingStrategy, ToolBasedStrategy>();

// Add health checks
builder.Services.AddHealthChecks()
    .AddCheck<McpServerHealthCheck>("mcp-servers");

var app = builder.Build();

// Map gateway endpoints
app.MapMcpGateway();
app.MapHealthChecks("/health");

// Start gateway
await app.RunAsync();
```

### Custom Routing Strategy

```csharp
public class CustomRoutingStrategy : IRoutingStrategy
{
    public string SelectServer(
        RoutingContext context,
        IEnumerable<ServerInfo> servers)
    {
        // Route admin clients to dedicated server
        if (context.ClientId.StartsWith("admin-"))
        {
            return servers.First(s => s.Name.Contains("admin")).Id;
        }

        // Route search tools to search-optimized server
        if (context.ToolName.StartsWith("search_"))
        {
            return servers.First(s => s.Name.Contains("search")).Id;
        }

        // Default: least connections
        return servers
            .OrderBy(s => s.ActiveConnections)
            .First()
            .Id;
    }
}
```

### Using the Gateway (Client Side)

```bash
# Route request through gateway
curl -X POST http://localhost:8080/mcp/invoke \
  -H "Content-Type: application/json" \
  -H "X-Client-Id: web-client" \
  -d '{
    "jsonrpc": "2.0",
    "id": "1",
    "method": "tools/call",
    "params": {
      "name": "search_documents",
      "arguments": {
        "query": "architecture"
      }
    }
  }'
```

---

## See Also

- [Gateway Architecture](../architecture.md)
- [Module Design](../module-design.md)
- [Routing Strategies](../routing-strategies.md)
- [Security Guide](../security.md)
- [AGENT-PRIMER.md](../../AGENT-PRIMER.md)

---

**Document Type:** API Reference
**Version:** 1.0.0
**Last Updated:** 2025-10-19
**Maintained By:** Svrnty Development Team